Secondary-Side LLC Resonant Controller IC With Dynamic PWM Dimming and Dual-Slope Clock Generator for LED Backlight Units

This paper presents a low-profile low-cost (LLC) resonant controller IC for LED backlight units fully operating at the secondary side. The integrated dimming circuitry is proposed to improve the dynamic current control characteristics and the LED current density for the brightness modulation of a large screen liquid crystal display. A dual-slope clock generator, including a soft start, is proposed in order to overcome the frequency error due to the undershoot found in conventional approaches. In addition, a new dead-time generator is proposed in order to implement an accurate dead time independent of the output frequency of the clock generator. Protection circuits, such as a under voltage lock out, thermal shut down, open LED detector, and shorted LED detector, have been implemented in order to improve the reliability of the controller IC. The chip is fabricated using 0.35 μm bipolar CMOS DMOS decimal technology; the die size is 2 mm × 2 mm. The frequency of the clock generator ranges from 50 to 500 kHz; the dead time ranges from 50 ns to 2.2 μs. The efficiency of the LED driving circuit is 91%. The current consumption of the LLC resonant controller IC is 40 mA for a 100 kHz operation frequency using a 15 V supply voltage.